Generally, a polarizing plate is used in a light control device using a liquid crystal cell. As this liquid crystal cell, for instance, a TN (Twisted Nematic) liquid crystal cell or a Guest Host (GH) liquid crystal cell is used.
FIGS. 12A and 12B are schematic drawings each showing the principle of an operation of a conventional light control device. This light control device is constituted mainly by a polarizing plate 1 and a GH cell 2. The GH cell 2, which is not shown in the drawings, is sealed between two glass substrates, and has a working electrode and a liquid crystal alignment film (this also applies to the following cases). A positive liquid crystal molecule 3 and a positive dichroic dye molecule 4 are sealed in the GH cell 2.
The positive dichroic dye molecule 4 has anisotropy of light absorption, and is, for example, a positive (p-type) dye molecule. Further, the positive liquid crystal molecule 3 is of the positive type (plus type), whose anisotropy of dielectric constant is positive.
FIG. 12A shows a state of the GH cell 2 during a voltage is not applied thereto (no voltage is applied thereto). Incident light 5 is transmitted by the polarizing plate 1 thereby to be linearly polarized. In FIG. 12A, this polarization direction coincides with a molecular long axis direction of the positive dichroic dye molecule 4. Thus, light is absorbed by the positive dichroic dye molecule 4, so that the light transmittance of the GH cell 2 is reduced.
Further, as shown in FIG. 12B, a voltage is applied to the GH cell 2. As the positive liquid crystal molecule 3 is directed toward the direction of an electrical field, the molecular long axis of the positive dichroic dye molecule 4 is perpendicular to the polarization direction of the linearly polarized light. Therefore, the incident light 5 is transmitted almost without being absorbed by the GH cell 2.
Incidentally, in the case of using a negative type (n-type) dichroic dye molecule, which absorbs light in a molecular short axis direction, conversely to the case of using the positive dichroic dye molecule 4, when no voltage is applied thereto, light is not absorbed, whereas light is absorbed when a voltage is applied thereto.
In the light control device shown in FIGS. 12A and 12B, the ratio of absorbance on application of a voltage to absorbance on application of no voltage, that is, an optical density ratio is about 10. This device has an optical density ratio that is about twice the optical density ratio of the light control apparatus constituted only by the GH cell 2 without using the polarizing plate 1.
On the other hand, ordinary video cameras and digital still cameras each have CCDs (Charge Coupled Devices) for converting the intensity of light into electrical signals. A single CCD has several hundred thousand to several million pixels.
Further, a color filter is provided corresponding to each of the pixels. For instance, in a case where a striped pattern or the like having a width, which is equal to that of this colored CCD pixel, is imaged, a part of color signals to be formed originally of three colors, that is, red, green, and blue is lacked. Thus, a color differing from an original color comes out. Also, a non-colored part is colored. Consequently, an image of the pattern or the like is very hard to see under such influences of false signals.
That is, the CCDs perform geometrically discrete sampling. This causes troubles that false color signals and moirés occur when geometrical patterns (of, for example, striped clothes, and tiled walls of buildings) finer than the periodic arrangement of the CCDs are shown, and that images of the patterns causes feeling of incongruity.
As a countermeasure thereagainst, recently, there has generally been employed means for preventing generation of false color signals by installing an optical lowpass filter, which is constituted by a birefringent plate made of quartz or the like, at a front position of the CCD thereby to blur high-frequency components of a striped pattern and so on and to make the striped pattern not to look like stripes and also make it clear which of a striped pattern or a color is shown.
Referring to FIGS. 14A and 14B, which illustrate a concrete principle, when natural light 31 having random oscillating directions is incident upon a birefringent plate 32 made of quartz or the like, the natural light 31 is split into an ordinary ray 33 and an extraordinary ray 34. Thus, a light ray forming an image at a single point is split to those respectively forming images at two points. A splitting axis d can be calculated according to the following equation (1). For instance, the splitting axis d is expressed as being about 5.9×10−3×t.
                    d        =                                                                              (                                      n                    e                                    )                                2                            -                                                (                                      n                    o                                    )                                2                                                    2              ⁢                                                n                  e                                ·                                  n                  o                                                              ×          t                                    equation        ⁢                                  ⁢                  (          1          )                    (incidentally, in the equation (1), t designates a thickness of the birefringent plate, and no denotes the refractive index of the ordinary ray, and ne designates the refractive index of the extraordinary ray).
For example, in a case where two birefringent plates differing in crystal axis from each other are combined with each other, as shown in FIG. 15A, what is called rhombic four-point blurring can be performed. In a case where three birefringent plates differing in crystal axis from one another, are combined with one another, as shown in FIG. 15B, what is called seven-point blurring can be performed. Also, in a case of a combination of three plates, in which a phase difference plate is sandwiched by birefringent plates, as shown in FIG. 15C, what is called square four-point blurring can be performed. Incidentally, because pixels are usually formed in a square arrangement in a digital still camera, the square four-point blurring shown in FIG. 15 has generally been employed. However, recently, with miniaturization of CCDs, employment of devices of the type having two birefringent plates made of quartz has been increased, in view of the balance between the necessity for enhancing the frequency characteristics and the cost thereof.
Additionally, regarding the digital cam coders, nearly similar optical lowpass filters have been employed.
However, it turns out that even in a case where the optical lowpass filter is disposed at a front position of the CCD, as shown in FIGS. 12A and 12B, and where an object, whose spatial separation capability is high, is imaged by using an imaging device, on which a light control device constituted by the GH cell 2 and the polarizing plate 1, as shown in FIGS. 12A and 12B, is mounted, a mode constituted by the GH cell 2 and the polarizing plate 1, the action of separation between an ordinary ray and an extraordinary ray due to birefringerence does not effectively function owing to the positional relation between the device and the optical axis of each of the birefringent plates, that a deviation of the intensity of the separated ray occurs, and that the effect of preventing the generation of a false color signal is insufficient. Improvement thereon has been desired.
The invention is accomplished to the above-mentioned problems. An object of the invention is to provide a light control device enabled to realize the enhancement of optical functions thereof, and to provide an imaging device enabled to realize the enhancement of the performance, the quality of picture, and the reliability thereof by disposing this light control device in an optical path thereof.